1. Field of the Invention
The present invention relates to a method and an arrangement for adjusting density in electrophotographic printing.
2. Description of the Related Art
Electrophotographic printing employs a developing process using a two-component developer, which is a mixture of toner particles and carrier particles. The two-component developer used most frequently is a mixture of toner particles and ferromagnetic carrier particles. Toner concentration (T/C) is a parameter of the developing process. The toner concentration is a mixture ratio between toner particles and carrier particles. Appropriate adjustment of the toner concentration is needed to keep density of electrophotographic image at a constant level.
Adjustment of the toner concentration may be accomplished by controlling toner supply in response to the result from comparing an actual value of sensed density of a toner mark with a reference value. FIG. 5 illustrates a known arrangement for adjusting density in electrophotographic printing. In FIG. 5, a toner mark sensor 31 senses density of a toner mark 36 developed on a recording paper 35 and produces a signal. An analog-to-digital (AD) converter 32 converts the sensor signal into a digital data. A controller 33 inputs the information of an actual value of density of the toner mark 36 from the digital data, compares the actual value of the density with a reference value and produces a toner sensor signal when the actual value is lower than the reference value. A toner supply 34 conducts toner supply in response to the toner sensor signal.
The following disclosures may be relevant to various aspects of the present invention:
JP-A 3-12669 Inventor: Kariya Published: Jan. 21, 1991
JP-A 10-39607 Inventor: Ohki Published: Feb. 13, 1998
JP-A 62-81648 Inventors: Yamada et al. Published: Apr. 15, 1987
JP-A 6-180534 Inventor: Mutou Published: Jun. 28, 1994
The relevant portions of the foregoing disclosures may be briefly summarized as follow:
JP-A 3-12669 discloses a laser printer employing a toner supply control. A toner mark sensor senses an actual value of density of a toner mark on each page or sheet that is defined between the adjacent two perforated creases of a continuous stationery. A toner supply conducts toner supply when the actual density of the toner mark is low. An air nozzle injects air toward the toner mark sensor upon resetting the printing paper.
JP-A 10-39607 discloses a toner supply control. A reference image with a predetermined density is developed on a photoconducive surface of a drum. A light-emitting diode (LED) projects a light onto the developed reference image on the drum. A photoreceptor receives the light reflected by the developed reference image and produces a signal indicative of density of the developed reference image. In response to this signal, fresh toner is fed to a developer. In order to prevent excessive enrichment of developer mixture, the fresh toner supply is interrupted when image forming condition is not complied with.
JP-A 62-81648 discloses a tractor unit for a laser printer. The tractor unit transfers a continuous stationery having perforated creases at regular spacing to define a page or sheet between the adjacent two perforated creases. In order to detect density of a toner mark developed on the continuous stationery outside of image forming area, a toner mark sensor is supported above side edge of the continuous business form by a sensor shifter. The sensor shifter cooperates with the tractor unit.
JP-A 6-180534 discloses a signal processing system for preventing excessive fresh toner supply by reducing a volume of fresh toner supply when a signal produced by a toner sensor exceeds a predetermined level.
In a toner supply control based on a signal of a toner mark sensor, a need remains to keep a distance between a toner mark sensor and a toner mark on each page of a continuous stationery constant. The continuous stationery has a number of perforated creases spaced equidistant along the longitudinal length of the paper to define pages, each between the adjacent two perforated creases. Prior to printing process, the continuous stationery is folded along its perforated creases. A toner mark is printed within an area of each page adjacent a leading one, with respect to a direction of movement of the continuous stationery, of two perforated creases which define the page. Immediately after unfolded along the perforated creases, two adjacent pages of the continuous stationery slope upwardly and downwardly toward their leading perforated creases. This may cause variations in distance between the toner mark sensor and a toner mark on each of the pages of the elongate printing page particularly if the continuous stationery is thick and less flexible.
An object of the present invention is to comply with the before mentioned need without any modification of hardware.